Add geodesic plugin.

geodesic/README.md

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+# Geodesic Grid
+
+Example: <http://bl.ocks.org/3057239>

geodesic/geodesic.js

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+(function() {
+  var φ = 1.618033988749895,
+      ρ = 180 / Math.PI;
+
+  var vertices = [
+    [1,φ,0], [-1,φ,0], [1,-φ,0], [-1,-φ,0],
+    [0,1,φ], [0,-1,φ], [0,1,-φ], [0,-1,-φ],
+    [φ,0,1], [-φ,0,1], [φ,0,-1], [-φ,0,-1]
+  ];
+
+  var triangles = [
+    [0,1,4],  [1,9,4],  [4,9,5],  [5,9,3],  [2,3,7],
+    [3,2,5],  [7,10,2], [0,8,10], [0,4,8],  [8,2,10],
+    [8,4,5],  [8,5,2],  [1,0,6],  [11,1,6], [3,9,11],
+    [6,10,7], [3,11,7], [11,6,7], [6,0,10], [9,1,11]
+  ].map(function(triangle) {
+    return triangle.map(function(i) {
+      return vertices[i];
+    });
+  });
+
+  d3.geodesic = function(s) {
+    return {
+      type: "MultiPolygon",
+      coordinates: geodesic(Math.round(s)).map(function(triangle) {
+        triangle = triangle.map(project);
+        triangle.push(triangle[2]);
+        return [triangle];
+      })
+    };
+  };
+
+  function geodesic(n) {
+    return d3.merge(triangles.map(function(triangle) {
+      var i01 = interpolate(triangle[0], triangle[1]),
+          i02 = interpolate(triangle[0], triangle[2]),
+          triangles = [];
+
+      triangles.push([
+        triangle[0],
+        i01(1 / n),
+        i02(1 / n)
+      ]);
+
+      for (var i = 1; i < n; ++i) {
+        var i1 = interpolate(i01(i / n), i02(i / n)),
+            i2 = interpolate(i01((i + 1) / n), i02((i + 1) / n));
+        for (var j = 0; j <= i; ++j) {
+          triangles.push([
+            i1(j / i),
+            i2(j / (i + 1)),
+            i2((j + 1) / (i + 1))
+          ]);
+        }
+      }
+
+      return triangles;
+    }));
+  }
+
+  function interpolate(p0, p1) {
+    var x0 = p0[0],
+        y0 = p0[1],
+        z0 = p0[2],
+        x1 = p1[0] - x0,
+        y1 = p1[1] - y0,
+        z1 = p1[2] - z0;
+    return function(t) {
+      return [
+        x0 + t * x1,
+        y0 + t * y1,
+        z0 + t * z1
+      ];
+    };
+  }
+
+  function project(p) {
+    var x = p[0],
+        y = p[1],
+        z = p[2];
+    return [
+      Math.atan2(y, x) * ρ,
+      Math.acos(z / Math.sqrt(x * x + y * y + z * z)) * ρ - 90
+    ];
+  }
+})();